This invention relates to a post-acceleration detector for a mass spectrometer, and more particularly to a post-acceleration detector of the kind described above which is capable of also detecting low-acceleration ions and high-mass ions.
Known publications disclosing prior art post-acceleration detectors for mass spectrometers include U.S. Pat. No. 4,267,448; a book entitled "Mass Spectrometry in Science and Technology" edited by F. A. White, published by John Wiley & Sons, Inc. pp. 101-109; JP-B2-58-7229; JP-B2-59-16706; JP-A-54-139592; JP-A-57-107550; JP-A-57-115752; JP-A-58-38446; JP-A-61-233958; JP-A-62-40147; JP-A-62-40148; and JP-A-63-276862.
According to the disclosures of these known publications, an ion beam, for example, a positive ion beam is directed to bombard a conversion electrode applied with a negative voltage thereby causing emission of negative charged particles from the bombarded surface of the conversion electrode, and the charged particles are then amplified by a secondary electron multiplier so as to detect an ionic current. Thus, when a considerably high voltage is applied to the conversion electrode, the negative charged particles can be accelerated up to a high velocity, so that the multiplication factor can be improved by the factor of several powers of ten as compared to the case where the ion beam is directly incident upon the secondary electron multiplier.
However, a change in the energy of the ion beam, that is, a change in the velocity of the ion beam results in a corresponding shift of the point of bombardment on the surface of the conversion electrode which is bombarded by the ion beam to emit the charged particles. More precisely, when the velocity of the ion beam becomes higher, the point of ion beam bombardment on the surface of the conversion electrode shifts away from an ion beam entrance slit, while when the velocity of the ion beam becomes lower, the point of ion beam bombardment on the surface of the conversion electrode shifts toward the ion beam entrance slit. Thus, a change in the velocity of the ion beam results in a corresponding shift of the point of ion beam bombardment on the surface of the conversion electrode, and the charged particles emitted from the bombarded surface of the conversion electrode may not be accurately directed toward the secondary electron multiplier.
In order to obviate such a defect, a method has been commonly employed hitherto in which the voltage applied to the conversion electrode is changed when the velocity of the ion beam changes, so that the ion beam can always accurately bombard the same point on the surface of the conversion electrode. The voltage applied to the conversion electrode is generally approximately proportional to the energy of the ion beam. Therefore, the voltage applied to the conversion electrode must be lowered when the energy of the ion beam becomes lower. However, when the voltage applied to the conversion electrode is lowered, the quantity of the charged particles emitted from the conversion electrode decreases, and the velocity of the emitted charged particles shows also a decrease. The resultant decrease in the multiplication factor of the secondary electron multiplier leads to difficulty of accurate detection of the ionic current.